Air flow system bypassing a balance in a model airplane being tested in a wind tunnel

Abstract

A model airplane for testing inside a wind tunnel has a fuselage and is mounted on a sting. A balance or force transducer is mounted on a forward end of the sting inside the fuselage of the model airplane and is of the strain gauge type for measuring forces acting on the model airplane. A plenum chamber for receiving air is mounted forwardly of the balance inside of the fuselage of the model airplane. At least one air feed line extends from the sting to the plenum chamber and bypasses or bridges over the balance. An external air source feeds air from the sting through the air feed line to the plenum chamber. Inside the plenum chamber, there is an externally pressurized bellow assembly having an internal cavity at a pressure equal to the static pressure existing in the fuselage of the model airplane. A fitting at the end of the bellow assembly allows air to enter the plenum chamber so that such below assembly is externally pressurized. Thus, by this arrangement, negligible force interactions from the air flowing in the sting, the air feed line, and the plenum chamber are transferred to the balance for measuring forces acting on the model airplane. To aid further in avoiding force interactions on the balance, a flexured bellow element is arranged in each air feed line so that undesirable torsion, pitch, and other forces are absorbed thereby and are prevented from being transferred to the balance.

I claim: 
     
       1. A model airplane having a fuselage for testing inside a wind tunnel, comprising: a sting;   an air source, connected to the sting, for feeding air through the sting;   a balance for measuring forces acting on the model airplane, said balance being mounted on the sting;   a plenum chamber for receiving air therein, said plenum chamber being mounted inside the fuselage in communication with the balance; and   at least one air feed line extending from the sting to the plenum chamber and bypassing the balance;   whereby negligible force interactions from air flowing in the sting, the at least one air feed line, and the plenum chamber are transferred to the balance for measuring forces.   
     
     
       2. The model airplane according to claim 1, further comprising: a flexured bellow element arranged in the at least one air feed line.   
     
     
       3. The model airplane according to claim 2, wherein: said flexured bellow element includes at least one flexure and at least one bellow.   
     
     
       4. The model airplane according to claim 1, further comprising: a bellow assembly mounted inside the plenum chamber and connected at one end to the at least one air feed line.   
     
     
       5. The model airplane according to claim 4, wherein: said beIlow assembly is externally pressurized by air in the plenum chamber and has an internal cavity at a static pressure existing inside the fuselage.   
     
     
       6. The model airplane according to claim 1, wherein: said at least one air feed line includes two air feed lines entering the plenum chamber from opposite sides.   
     
     
       7. The model airplane according to claim 6, further comprising: a matched pair of bellow assemblies mounted inside the plenum chamber, each of said matched pair of bellow assemblies being connected at one end of one of the two air feed lines entering the plenum chamber from opposite sides.   
     
     
       8. The model airplane according to claim 7, further comprising: a central fitting arranged in the plenum chamber and connecting opposite ends of each of the matched pair of bellow assemblies to each other.   
     
     
       9. The model airplane according to claim 8, further comprising: means, arranged on the central fitting, for externally pressurizing the matched pair of bellow assemblies.   
     
     
       10. The model airplane according to claim 1, further comprising: port means, arranged in the plenum chamber, for allowing a flow of air to exit from the plenum chamber.   
     
     
       11. The model airplane according to claim 10, further comprising: valve means, connected to the port means, for managing the flow of air exiting from the plenum chamber.   
     
     
       12. A model airplane having a fuselage for testing inside a wind tunnel and being adapted to be mounted to a sting with an air source connected thereto for feeding air therethrough, said model airplane comprising: a balance for measuring forces acting on the model airplane, said balance being mounted on the sting;   a plenum chamber for receiving air therein, said plenum chamber being mounted inside the fuselage in communication with the balance; and   at least one air feed line extending from the sting to the plenum chamber and bypassing the balance;   whereby negligible force interactions from air flowing in the sting, the at least one air feed line, and the plenum chamber are transferred to the balance.   
     
     
       13. The model airplane according to claim 12, further comprising: a flexured bellow element arranged in the at least one air feed line.   
     
     
       14. The model airplane according to claim 13, wherein: said flexured bellow element includes at least one flexure and at least one bellow.   
     
     
       15. The model airplane according to claim 12, further comprising: a bellow assembly mounted inside the plenum chamber and connected at one end to the at least one air feed line.   
     
     
       16. The model airplane according to claim 15, wherein: said hollow assembly is externally pressurized by air in the plenum chamber and has an internal cavity at a static pressure existing inside the fuselage.   
     
     
       17. The model airplane according to claim 12, wherein: said at least one air feed line includes two air feed line entering the plenum chamber from opposite sides.   
     
     
       18. The model airplane according to claim 17, further comprising: a matched pair of bellow assemblies mounted inside the plenum chamber, each of said matched pair of bellow assemblies being connected at one end of one of the two air feed lines entering the plenum chamber from opposite sides.   
     
     
       19. The model airplane according to claim 18, further comprising: a central fitting arranged in the plenum chamber and connecting opposite ends of each of the matched pair of bellow assemblies to each other.   
     
     
       20. The model airplane according to claim 19, further comprising: means, arranged on the central fitting, for externally pressurizing the matched pair of bellow assemblies.